The use of complex electronic systems in automobiles has increased dramatically over the past several years. Radar systems have been used in advanced cruise control systems, collision avoidance systems, and hazard locating systems. For example, systems are available today that inform the driver if an object (e.g. child's bicycle, fire hydrant) is in the vehicle's path even if the object is hidden from the driver's view.
Systems such as these utilize small radar sensor modules that are mounted somewhere on the automobile (e.g., behind the front grill, in the rear bumper). The module contains one or more antennas for transmitting and receiving radar signals. These devices work by transmitting radio frequency (RF) energy at a given frequency. The signal is reflected back from any objects in its path. If any objects are present, the reflected signal is processed and an audible signal is sounded to alert the driver. One example of this type of radar system is the 24 GHz High Resolution Radar (HRR) developed by M/A-Com Inc. (Lowell, Mass.).
The radar sensor units used in these systems typically utilize two independent antenna arrays. A first array is used to transmit the outbound signals, and a second antenna array is used to receive the reflected return signals. The two antenna arrays are formed on a single substrate and are generally separated by a space of three to four inches.
Microstrip antenna arrays are often used in this type of application because they have a low profile and are easily manufactured at a low cost. In addition, microstrip antenna arrays are versatile and can be used in applications requiring either directional or omni-directional coverage. Microstrip antenna arrays operate using an unbalanced conducting strip suspended above a ground plane. The conductive strip resides on a dielectric substrate. Radiation occurs along the strip at the points where the line is unbalanced (e.g., corners, bends, notches, etc.). This occurs because the electric fields associated with the microstrip along the balanced portion of the strip (i.e., along the straight portions) cancel one another, thus removing any radiated field. However, where there is no balance of electric fields, radiation exists. By controlling the shape of the microstrip, the radiation properties of the antenna can be controlled.
Slot-coupled microstrip antennas arrays comprise a series of microstrip patch antennas that are parasitically coupled to a feed microstrip. The feed microstrip resides below the ground plane and is coupled to each of the patch microstrips through a slot in the ground plane. Various numbers of patch antennas can be coupled to a single microstrip input feed to form the array. Six-element arrays and eight-element arrays are commonly used in High Resolution Radar (HRR) sensors, although any number of patch elements can be coupled to the feed microstrip.
One problem that arises using this type of antenna design is that the transmit and receive antenna arrays are not perfectly isolated from each other. There is some level of RF signal leakage between the two antenna arrays, either through the air or through the substrate material. The leakage through the substrate is caused by undesired surface wave propagation. This coupling effect between the two antenna arrays lowers antenna gain and reduces performance of the radar sensor.
Presently, several techniques are used to improve isolation between microstrip array antennas. Two techniques are shown in FIG. 1. The first technique, shown in FIG. 1a, involves placing a metal wall 11 in the antenna unit 10 between the transmitting antenna array 13 and receiving antenna array 15. The metal wall 11 improves the isolation between the two microstrip array antennas by blocking or reflecting back signals passing through the air within the cavity 17 formed within the antenna unit 10. While using a wall 11 such as this will improve isolation between the two antennas, it has several drawbacks. First, the addition of a metal wall 11 in the antenna unit 10 consumes additional space and is cumbersome. As antenna units are becoming increasingly smaller, it is undesirable to introduce an additional space consuming component. Secondly, the isolation achieved by inserting the metal wall 11 is not as high as desired (only about 4 dB improvement in the isolation is obtained). Much of the signal leakage occurs through the substrate rather than by radiated signals traveling through the air within the antenna unit 10. The metal wall 11 does not sufficiently block any signal coupling which occurs via the substrate layer.
A second technique used to provide isolation is illustrated in FIG. 1b. This technique involves placing a section 12 of a signal absorbing material in the cavity 18 formed between the transmitting antenna 14 and the receiving antenna 16 within the antenna unit 20. For example, a section 12 of Eccosorb GDS sheet (Emerson & Cuming Microwave Products, Inc., Randolph, Mass.) can be placed between the antennas to absorb radiation within the unit 20 and thus improve isolation between the antennas. However, this technique also has limitations. While the absorbing materials such as Eccosorb GDS provide an improvement in isolation over the metal wall (about 8 dB improvement in the isolation is obtained), the isolation is not as complete as desired. In addition, the absorbing materials are high in cost.
Despite attempts to improve isolation between antennas within an antenna unit using these techniques, often the level of isolation achieved proves to be insufficient. Accordingly, there is a need for an antenna unit that provides a high level of isolation between the antennas, while at the same time is compact, cost efficient, and achieves a high level of gain. The present invention fulfills these needs among others.